Capacitive Sensing Array Modulation

ABSTRACT

A capacitive fingerprint sensor that may be formed of an array of sensing elements. Each capacitive sensing element of the array may register a voltage that varies with the capacitance of a capacitive coupling. A finger may capacitively couple to the individual capacitive sensing elements of the sensor, such that the sensor may sense a capacitance between each capacitive sensing element and the flesh of the fingerprint. The capacitance signal may be detected by sensing the change in voltage on the capacitive sensing element as the relative voltage between the finger and the sensing chip is changed. Alternately, the capacitance signal may be detected by sensing the change in charge received by the capacitive sensing elements as the relative voltage between the finger and the sensing chip is changed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/842,635, filed on Mar. 15, 2013, and entitled “Capacitive SensingArray Modulation,” which claims the benefit under 35 U.S.C. §119(e) toU.S. Provisional Patent Application No. 61/623,941, filed on Apr. 13,2012, and entitled “Capacitive Sensing Array Modulation,” U.S.Provisional Patent Application No. 61/649,203, filed on May 18, 2012,and entitled “Inductive Boost Circuit and Fingerprint Sensing Systems,”U.S. Provisional Patent Application No. 61/666,579, filed on Jun. 29,2012, and entitled “Capacitive Sensing Array Modulation,” and U.S.Provisional Patent Application No. 61/666,610, filed on Jun. 29, 2012,and entitled “Capacitive Sensing Array,” all of which are incorporatedby reference as if fully disclosed herein.

TECHNICAL FIELD

Embodiments described herein relate generally to circuits that may beused to support fingerprint sensing, including boost circuits, such asinductive boost circuits.

BACKGROUND DESCRIPTION

The following terminology is exemplary, and not intended to be limitingin any way. The text “capacitive sensing element”, and variants thereof,generally refers to one or more data elements of any kind, includinginformation sensed with respect to individual locations. For example andwithout limitation, a capacitive sensing element can include data orother information with respect to a relatively small region of afingerprint image. After reading this application, those skilled in theart would recognize that these statements of terminology would beapplicable to techniques, methods, physical elements, and systems(whether currently known or otherwise), including extensions thereofinferred or inferable by those skilled in the art after reading thisapplication.

Fingerprint sensing technology has become widespread in use and is oftenused to provide secure access to sensitive electronic devices and/ordata. Generally, capacitive fingerprint sensors may be used to determinean image of a fingerprint through measuring capacitance through eachcapacitive sensing element of a capacitive sensor. The higher thecapacitance, the nearer the surface of an adjacent or overlying fingerto the capacitive sensing element. Thus, fingerprint ridges provide ahigher capacitance in an underlying capacitive sensing element than dofingerprint valleys.

Capacitive fingerprint sensors come in at least two varieties, namelyactive and passive. Active capacitive sensors are often used inelectronic devices to provide biometric security and identification ofusers.

Active capacitive sensors initially excite the epidermis of the sensedfinger. Capacitance to the epidermis is measured at each capacitivesensing element. As one example, capacitance may be measured ordetermined by measuring a capacitive sensing element's voltage and/orcharge during a low voltage phase and a high voltage phase of amodulation frequency for the capacitive sensing element array. Thedifference in voltages may be used to determine capacitance. One exampleof an active capacitive sensor is shown in FIG. 1.

As shown in FIG. 1, the active fingerprint sensor may include bothcapacitive sensing element array 102 on sensor chip 100 and drive ring104. The voltage of capacitive sensing element array 102 is not directlydriven or modulated, but instead drive ring 104 is modulated by driveamplifier 106. This, in turn, excites finger 108 and the voltage and/orcharge at each capacitive sensing element of capacitive sensing elementarray 102 varies as drive ring 104 is modulated since finger's 108voltage potential changes with the modulation of drive ring 104.

In such a sensor, the voltage that may be applied to the drive ring maybe limited. Commonly, the drive ring voltage is no more than 4 voltspeak-to-peak. Voltages above this may excite the finger to too high avoltage; this excessive excitation may be detected by a person as a“tingling” or uncomfortable feeling in their finger. Although the exactvoltage at which one can sense the tingling varies from person toperson, a 4 volt peak-to-peak voltage is generally considered as thethreshold beyond which the feeling is noticeable.

Since the drive ring's voltage is restricted to avoid user perception,the thickness of any dielectric overlaying the sensor may also belimited. The thicker the dielectric between sensor pad and finger, themore attenuated the resulting capacitance and the blurrier thefingerprint image becomes. For dielectrics having a thickness or morethan approximately 100 microns, the fingerprint image may becomeunreliable.

Another limitation arises when other parts of the user's finger or handor body may capacitively couple through earth ground to the system, ordirectly to the system ground when touching other parts of the system.This capacitive coupling from the user to the system may be highlyvariable depending on how the user is touching the device. Thisparasitic coupling attenuates the voltage that the drive ring is abledrive into the user's finger, and as such reduces the signal. Theattenuation may be highly variable depending on how the user is touchingthe device.

SUMMARY

Embodiments described herein may take the form of an electronic device,including: a sensor pad comprising an array of individual capacitivesensing elements; a drive ring connected to the sensor pad; and amodulating circuit adapted to control a modulated signal received by thesensor pad while a drive signal received by the drive ring is maintainedsubstantially at a boosted ground exceeding an electronic device groundlevel.

Still other embodiments may take the form of an electronic device,including: a capacitive sensor comprising: a sensor pad comprising anarray of individual capacitive sensing elements; and a drive ringconnected to the sensor pad; a first modulating circuit operablyconnected to the sensor pad, wherein the first modulating circuit isadapted to modulate an input signal to the sensor pad while a drivesignal to the drive ring is maintained substantially at a boosted groundexceeding an electronic device ground level; an inductive boost circuitconfigured to provide a boosted voltage to the sensor pad relative tothe electronic device ground level at a higher amplitude than therelative voltage between the drive ring and the electronic deviceground; and a housing surrounding the first modulating circuit andinductive boost circuit.

Yet other embodiments may take the form of a method for operating acapacitive sensor, wherein the capacitive sensor includes a sensor padcomprising an array of individual capacitive sensing elements and adrive ring connected to the sensor pad, the method comprising: during ascanning state, modulating one or more input signals to the sensor padwhile maintaining a drive signal to the drive ring substantially at asystem ground level; and during an idle state, maintaining the one ormore input signals to the sensor pad substantially at a boosted groundexceeding the system ground level.

Still other embodiments may take the form of a method for operating acapacitive sensor, wherein the capacitive sensor includes a sensor padcomprising an array of individual capacitive sensing elements and adrive ring operably connected to the sensor pad, the method comprising:initiating a scanning operation and transitioning a drive signal to thedrive ring to a system ground level; reading a first signal from acapacitive sensing element when an input signal to the sensor pad is ina low state and the drive signal to the drive ring is maintainedsubstantially at the system ground level; and reading a second signalfrom the capacitive sensing element when the input signal to the sensorpad is in a high state and the drive signal to the drive ring ismaintained substantially at a boosted ground exceeding the system groundlevel.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a block diagram of a sample capacitive sensing array.

FIG. 2 depicts a block diagram of an embodiment of a capacitive sensingarray, as described herein.

FIG. 3 depicts a sample electronic device incorporating the embodimentof a capacitive sensing array.

FIG. 4 is a schematic illustration of a fingerprint sensing systemaccording to an embodiment.

FIG. 5 is a schematic illustration of an inductive boost circuit inaccordance with an embodiment, together with a timing diagram showingexample operation of the inductive boost circuit.

FIG. 6 is a schematic illustration of another fingerprint sensing systemin accordance with an embodiment.

DETAILED DESCRIPTION

Generally, embodiments discussed herein may take the form of acapacitive sensor, such as a fingerprint sensor. The sensor may beformed from a capacitive sensing element array; each capacitive sensingelement of the array may register a voltage that varies with thecapacitance of a capacitive coupling. A finger may capacitively coupleto the individual capacitive sensing elements of the sensor, such thatthe sensor may sense a capacitance between each capacitive sensingelement and the flesh of the fingerprint. The capacitance signal may bedetected by sensing the change in voltage on the capacitive sensingelement as the relative voltage between the finger and the sensing chipis changed. Alternately, the capacitance signal may be detected bysensing the change in charge received by the capacitive sensing elementsas the relative voltage between the finger and the sensing chip ischanged. Portions of the finger further away from the sensor may createa lower capacitance between the sensor and finger, and thus a lowersignal on underlying capacitive sensing elements. By contrast, portionsof the finger closer to the sensor may create a higher capacitancebetween sensor and finger, and thus higher signals on underlyingcapacitive sensing elements. Thus, capacitive sensing elementsunderlying ridges of a fingerprint may register higher signals whilecapacitive sensing elements underlying valleys of the fingerprint mayregister a lower capacitance and lower signals.

The fingerprint sensor may include both a sensor pad and a drive ring.Both the sensor pad and drive ring may be placed beneath a dielectric,such that the finger does not directly touch either the sensor pad ordrive ring but instead comes into contact with the interposeddielectric. In some examples, the finger may contact the drive ring.

The difference in signals at the sensor capacitive sensing elements maybe used to map a fingerprint. The resolution of the sensor may vary withcapacitive sensing element density, distance between the sensor pad'ssurface and the finger, and thickness of a dielectric covering thesensor pad. Generally, as the dielectric increases in thickness, thecapacitance between the finger and sensor grows increasingly attenuated,and so the signal differences may attenuate, as well. Certainembodiments may address this attenuation by applying a higher, lessvariable, relative voltage change between the finger and the sensor pad,leading to a higher detectable signal at the sensor, as described inmore detail herein.

By driving a higher voltage to the sensor, the capacitive couplingbetween the sensor and finger may compensate for the thickness of thedielectric. By increasing the voltage in this fashion, the capacitancebetween finger and capacitive sensing elements of the sensor pad may beincreased, thereby obtaining better resolution and imaging of thefingerprint. The sensor may be driven at a higher voltage, such as 12volts peak-to-peak, 16 volts peak-to-peak, or even more, withoutinducing any physical sensation in the finger or hand of a user. Thismay be accomplished by maintaining the drive ring's voltage at a systemground voltage, while the sensor array is modulated with the higherpeak-to-peak voltage signal.

By driving a voltage to the sensor relative to system ground, whilemaintaining the drive ring's voltage at system ground, minimizes theissue of signal attenuation due to the highly variable capacitivecoupling between the user and the system ground as a result of otherfingers, hands, or body parts coming in contact with other parts of thedevice.

FIG. 2 depicts one sample embodiment of a fingerprint sensor. Sensor 200may include sensor pad 202 formed by an array of individual capacitivesensing elements. Although sensor pad 202 is shown, the individualcapacitive sensing elements are omitted for purposes of clarity. Aspreviously mentioned, each capacitive sensing element may becapacitively coupled to a portion of finger 212 overlying thatcapacitive sensing element; the distance between the overlying fingerportion and capacitive sensing element determines the capacitancebetween the two and thus the signal registered at the capacitive sensingelement. As distance decreases, signal increases. Each capacitivesensing element is also capacitively coupled to other adjacent overlyingfinger portions, and as the distance increases, this leads to a blurringeffect, which reduces the total signal difference between capacitivesensing elements directly under finger ridges and finger valleys.

The fingerprint sensor may also include drive ring 204. Drive ring 204may be maintained at a system ground voltage, rather than an absolute orearth ground.

The fingerprint sensor may also include first and second drive amplifier206 and 208, as depicted in FIG. 2. First drive amplifier 206 may drivea sensor integrated circuit that includes sensor pad 202, such as anapplication-specific integrated circuit (ASIC) with a modulatingvoltage, to induce relative voltage between sensor pad 202 and userfinger 212. Second drive amplifier 208 may, provide voltage and currentto the sensor integrated circuit, so that the sensor integrated circuitcan operate from the relative voltage difference between first andsecond drive amplifiers 206 and 208. Second drive amplifier 208 maysupply a modulating voltage in sync with first drive amplifier 206, suchthat the relative voltage between the outputs of the first and secondamplifiers is approximately constant. The first and second driveamplifiers may be connected by capacitor 210, which can help amplifiers206 and 208 keep the relative voltage between the first and second driveamplifier outputs constant .

FIG. 3 depicts electronic device 300 that may incorporate a fingerprintsensor, such as that shown in FIG. 2. Electronic device 300 may be amobile telephone, a tablet computing device, a notebook computer, apersonal digital assistant, a desktop computer, a portable media player,and the like. The sensor pad may be placed below an input mechanism,such as button 302 of electronic device 300. The sensor pad mayalternatively be placed beneath a portion of display screen 304 ofelectronic device 300, beneath sidewall 306 or other portion of thedevice's enclosure, and the like. Essentially, any portion of theelectronic device's enclosure may house the fingerprint sensor.

In some embodiments, a fingerprint sensor chip (including both sensorpad and drive ring) may be positioned beneath the button 302. An inklayer and/or adhesive may be placed between the button's bottom surfaceand the sensor chip's top surface. The adhesive may bond the chip to thebutton, for example. One or more solder balls may affix the fingerprintsensor chip to a flex conductor. The solder balls may generally beplaced near the center of the fingerprint sensor chip to reduce thelikelihood of cracking due to stress.

In still other embodiments, a number of fingerprint sensors as describedherein may be employed in a single electronic device. For example, thefingerprint sensors may be arranged beneath a cover glass or outersurface of an electronic device. In some embodiments, the sensors may bearranged in an array or other ordered pattern. In other embodiments, thesensors may be placed randomly or semi-randomly beneath the surface. Instill other embodiments, the sensors may be placed within or beneath adisplay stack of an electronic device incorporating a display, such as amobile phone, tablet computing device, computer display screen, and thelike. In such embodiments, the sensor(s) may capacitively sense afingerprint through the display and/or cover glass of the display.

Further, in such embodiments, a portion of the device enclosure or framemay serve as a ground ring as described herein. Multiple sensors mayshare a ground ring in such a configuration. For example, if the mobiledevice may capacitively sense touch, the device may activate afingerprint sensor at, under or near the location at which a touch wassensed. In some embodiments, only fingerprint sensors corresponding tothe touch location may be activated while others remain inactive. Theground ring (e.g., portion of the housing, such as sidewall 306) may becoupled to the active fingerprint sensor to obtain a fingerprint orpartial fingerprint and operate as generally described herein.

As yet another option, one or more fingerprint sensors may be placedoutside of the display area 304 but beneath a surface of the electronicdevice 300. For example, one or more sensors may be located in a chin(e.g., a region below the display 304, such as the area around button302) or a forehead (e.g., a region above the display 304) of theelectronic device 300.

In some embodiments, flex circuit may extend from the input mechanismstack-up and connect the sensor chip to a signal provider chip that mayfacilitate modulating the voltage and/or operating frequency of thefingerprint sensor chip. In some examples, the signal provider chip andfingerprint sensor chip may be provided as a single chip or distributedamongst a different number of chips. External components may also beused. As one example, an ASIC may be used as the signal provider chip.The ASIC may also communicate data to and from a processor of theelectronic device.

FIG. 4 is a schematic illustration of a fingerprint sensing systemaccording to an embodiment. The fingerprint sensing system 400 includesa fingerprint sensor chip 405 and a signal provider chip 410. A drivering 412 is also shown. During operation, the sensor provided by thefingerprint sensor chip may be modulated using a higher voltage, e.g.16V peak-to-peak, while the drive ring 412 may be held at a systemground voltage. The system ground voltage may be provided by the signalprovider chip 410 and is shown as NRING in FIG. 4. The 16V peak-to-peaksignal may be provided by the signal provider chip 410, and is shown asNGND in FIG. 4.

General operation of the fingerprint sensing system shown in FIG. 4 willnow be discussed. It is to be noted that in alternative embodimentscertain components shown in FIG. 4 may be omitted, others added, orboth. The distribution of components as between external components, thefingerprint sensor chip, and the signal provider chip may also bedifferent in other embodiments. Conceptually, a sensor pad (not shown inFIG. 4) may be integrated into or coupled to the fingerprint sensor chip405. The capacitive sensing element array may capacitively couple to afinger of a user's hand.

The capacitive sensing element array may be modulated at a variety offrequencies. The capacitive sensing element array may measure thevoltage of all capacitive sensing elements during a high voltage andagain during a low voltage. That is, voltages supplied to the capacitivesensing element array and/or the sensor chip ground voltage, maygenerally resemble a square wave, although in alternative embodimentsthe voltage may take other wave shapes. A modulator (labeled “mod.block” in FIG. 4) may control the modulation or oscillation of thecapacitive sensing element array and/or the sensor chip ground voltagein accordance with a clock signal, NDRV in FIG. 4. The differencebetween the capacitive sensing element voltages and/or charges, asmeasured during the high and low portions of the modulation cycle,permits the chip to determine the capacitance between each capacitivesensing element and overlying portion of the finger and thus thecapacitance. From the capacitance with each capacitive sensing element,the fingerprint may be sensed, measured and reconstructed as known tothose skilled in the art and generally discussed above.

The signal provider chip 410 may provide a modulated high voltage signal(e.g. 16V peak-to-peak) to the fingerprint sensor chip 405 for use inmodulating the sensor. While 16V is used here as an example, in otherexamples different voltages may be used, including voltages greater than4V in some examples, greater than 6V in some examples, greater than 8Vin some examples, greater than 10V in some examples, greater than 12V insome examples, greater than 14V in some examples, and greater than 16Vin some examples. The signal provider chip 410 may further provide aground voltage, shown as NRING in FIG. 4 to the drive ring 412. Othervoltages for NRING may be used in other examples, including 4V, lessthan 4V, less than 2V, or negative voltages. In some examples, the drivering 412 may be kept at a voltage that may not be felt by a usertouching the drive ring 412 but that has a sufficient difference inmagnitude from the voltage provided to the sensor (e.g. NGND) to providesuitable signal-to-noise ratio for fingerprint sensing. In one example,that difference in magnitude is 16V, with the sensor voltage providedbeing 16V and the drive ring held at a ground voltage.

Example components of the fingerprint sensing system 400 used to form aninductive boost circuit to provide the NGND signal (which may be, e.g.16V) will now be described. Reference will be made to transistors 421,422, and 423, inductor 425, timing block 427, diode 442, diode 443, andlevel translators 430. A clock signal, e.g. NDRV of FIG. 4, may bereceived by the timing block 427. The clock signal may be provided to alevel translator 430 prior to receipt by the timing block 427 in someexamples. The level translator 430 may function to change a magnitude ofthe clock signal in some examples where, e.g. the timing block mayoperate in a different supply power domain than the fingerprint sensorchip 405. The timing block 425 may output respective versions of theclock signal (e.g. NDRV) to the gates of the transistors 421, 422, and423. The respective versions of the clock signals may be delayed varyingamounts as applied by the timing block 427. For example, the timingblock may include one or more delay circuits configured to delay theclock signal (e.g. NDRV) prior to providing a delayed signal to thetransistors 421, 422, and/or 423.

The transistor 423 may serve as a pass transistor. For example, thetransistor 423 may turn on when a clock signal, e.g. NDRV, is providedto the timing block 427. Accordingly, the signal provided by the timingblock to a gate of the transistor 423 may be indicative of the presenceor absence of a clock signal (e.g. NDRV). In this manner, the transistor423 may be turned on when the fingerprint sensor chip 405 is active andmay be turned off when the fingerprint sensor chip 405 is inactive. Thesignal provided by the timing block 427 to the gate of the transistor423 may be provided through an optional gate driver circuit as shown.When the transistor 423 is turned on, a supply voltage (e.g. 1.8 V insome examples) may be provided to the inductor 425, to allow theinductor 425 to store energy. The inductor may charge capacitor 432 to aboosted voltage, e.g. a voltage greater than the power supply voltage.In some examples, the inductor provides a boosted voltage of 16 V. Insome embodiments, capacitor 432 can be implemented as a physicalcomponent of the system, in some embodiments the capacitor 432 may beimplemented as the parasitic capacitance of the fingerprint sensor 405to the environment and the system ground, and combinations of these maybe used in some embodiments.

The transistor 421 may function to chop a voltage provided by theinductor 425 at a frequency expected by the fingerprint sensor chip 405,such as 2 MHz in some examples. In some examples, the transistor 421 maychop the voltage at a same frequency as the clock signal, e.g. NDRV.When the transistor 421 is off, the voltage NGND may be a boosted powersupply voltage developed by the inductor 425, e.g. 16V. When thetransistor 421 is on, the current flowing in the inductor 425 may find apath to ground, and the voltage NGND may transition to ground.Accordingly, the transistor 421 may be switched at a particularfrequency to achieve a shaped 16V peak-to-peak waveform. Other peakvoltages may also be used. In some examples, the switched frequency isthe same as the frequency of the clock signal, e.g. NDRV. Accordingly,the timing block 427 may delay the clock signal, e.g. NDRV, a particularamount and provide a delayed clock signal (e.g. delayed NDRV) to a gateof the transistor 421. The amount of delay may be related to the desiredNGND voltage. In one example, the delay is 25 ns when a 2 MHz clock isused. In this manner, the NGND signal provided at the node labeled NGNDmay be a boosted (e.g. 16 V) square wave signal at a frequency of 2 MHz.Other frequencies and voltages may be used.

In some examples, it may be disadvantageous to have the inductor 425coupled directly to the node providing NGND. For example, the inductor425 may then be coupled to parasitic capacitances, shown in FIG. 4 asC_(para) 432. The parasitic capacitances may represent, for example,capacitance between the fingerprint sensor chip 405 and ground, and mayinclude capacitances through a body of the user touching the fingerprintsensor. When the inductor 425 is coupled to the parasitic capacitance432, oscillation of the boosted voltage (e.g. NGND) signal may occur. Tominimize or remove oscillations, the transistor 422 may be provided.When turned on, the transistor 422 may advantageously divert the currentfrom the inductor 423 to damp oscillations which otherwise may occur.Accordingly, the timing block 427 may provide a version, which may be adelayed version, of the clock signal (e.g. NDRV) to the gate of thetransistor 422. The clock signal (e.g. NDRV) may be delayed by thetiming block 427 an amount sufficient such that when the transistor 422turns on, the voltage NGND has already reached the target voltage (e.g.16V).

In this manner, an inductive boost circuit may be used to provide aboosted voltage (e.g. 16 V) at the node labeled NGND in FIG. 4 (afterthe diode 443). The boosted voltage may be provided to the fingerprintsensor chip 405 for use in modulating the sensor. The boosted voltageitself may be provided as a modulated signal having a frequency, whichmay be the same as a frequency of the system clock signal (e.g. NDRV).

The low dropout regulator (LDO) 450 may provide a constant supplyvoltage to the fingerprint sensor chip (e.g. 1.8V). This voltage isprovided respectively to the node NGND which can be at the system groundvoltage or the boosted voltage.

The four communication lines labeled SCLK, MOSI, MISO and INT generallypermit data to be transferred to and from the fingerprint sensor chip.The signals may be provided through the level translator 430. In thefingerprint sensor chip 405, the communication lines are referenced tothe sensor ground NGND that is different from the system ground. Thelevel translator may allow for communication to happen between the NGNDreferenced fingerprint sensor chip and the system ground (GND)referenced device processor. The communication may happen synchronouslyor asynchronously with the modulation signal NDRV. During thecommunication, the node NGND can be at the system ground voltage or atthe boosted voltage.

SCLK is the serial communication clock and may be used for timingoperations.

The MISO line may be used for master/slave transmission of data from aslave to a master. In the present embodiment, the fingerprint sensorchip may be the slave, while a processor associated with the electronicdevice incorporating the sensor may be the master. Likewise, the MOSIline may be used for sending data from the master (e.g., deviceprocessor) to the slave (e.g., fingerprint sensor chip).

The INT line may be used for interrupts. The fingerprint sensor chip mayuse the INT line to provide a signal indicating to the device processorthat the fingerprint sensor chip has completed a task or that thefingerprint sensor chip has some fingerprint data ready to betransferred.

During an idle state, the fingerprint sensor chip 405 may operate in arelatively low-power state. When the chip detects a finger positionedabove the capacitive sensing element array, it may initiate a scanningoperation. The chip may detect a nearby finger by measuring acapacitance change of one or more capacitive sensing elements of thearray relative to its ground, as one example. This may cause the chip tobegin a scanning operation. The scanning operation generally may includereading the voltage or charge of one or more of the capacitive sensingelements during a low-voltage state of the array or sensor chip ground,driving the array or sensor chip ground to a high voltage and againreading the voltage or charge of one or more of the capacitive sensingelements. The scanning operation may alternately include reading thevoltage or charge of one or more of the capacitive sensing elementsduring a high-voltage state of the array or sensor chip ground, drivingthe capacitive sensing element array or sensor chip ground to a lowvoltage and again reading the voltage or charge of one or more of thecapacitive sensing elements. The capacitive sensing elements may bemodulated at a relatively high frequency between the low and highvoltage states by the modulator. In some embodiments, the chip mayoperate at a two megahertz frequency, although other embodiments mayemploy higher or lower frequencies.

It should be appreciated that the drive ring may not be modulated withthe capacitive sensing element array. Rather, the drive ring's potentialmay remain constant at system ground or very near to system ground. Thismay help prevent or reduce excitation and/or modulation of the finger,which in turn permits the use of higher voltages to drive the capacitivesensing element array.

Further, in embodiments where the fingerprint sensor is incorporatedinto a hand-held device such as the mobile device of FIG. 3, otherfingers of the user's hand may be in contact with a metal portion, orother conductive portion, of the enclosure. The enclosure generally isalso at system ground, so contact between conductive elements on theenclosure and the user's hand facilitate driving or maintaining thevoltage of the sensed finger to system ground. The “sensed finger” isthe finger overlaying the sensor pad, or the finger being imaged by thefingerprint sensor. Maintaining the sensed finger at or near systemground may enhance resolution of the fingerprint image and assist inpreventing modulation of the finger.

FIG. 5 is a schematic illustration of an inductive boost circuit inaccordance with an embodiment, together with a timing diagram showingexample operation of the inductive boost circuit. The inductive boostcircuit 500 is the same used in the fingerprint sensing system 400 ofFIG. 4, and like components are labeled with like reference numbers, thedetails of which are not repeated here for brevity. Operation of theinductive boost circuit 500 will be described with reference to thetiming diagram shown in FIG. 5 for added clarity. As discussed above,the transistor 423 may function as a pass transistor that may be onwhenever an active clock signal (e.g. NDRV) is present. Accordingly, forthe purposes of discussion of FIG. 5, the transistor 423 is assumed ONduring operations described with reference to FIG. 5. Similarly, thediode 442 may be used only when transistor 423 turns off following afingerprint scan. Accordingly, the diode 442 is assumed OFF duringdescribed operations with reference to FIG. 5.

At time t₀, the transistor 421 may turn off (e.g. the gate voltage shownin the trace related to transistor 421 of FIG. 5 may transition low,turning the transistor 421 off). When the transistor 421 is turned off,the inductor current I_(L) decreases as the voltage at node NGND beginsto rise to the boosted voltage (e.g. 16V) as shown in the timing diagramtraces labeled I_(L) and NGND. The voltage of NGND may peak at time t₁,as shown, where t₁ may represent an inductor voltage buildup timefollowing t₀. At time t₁, then, the diode 443 may turn off, not allowingcurrent to flow through the diode 443 to capacitor 432, and thusmaintaining the voltage on NGND.

At time t₂, the transistor 422 may turn on (e.g. the gate voltage shownin the trace related to transistor 422 of FIG. 5 may transition high,turning the transistor 422 on). This may allow damping through thetransistor 422 to prevent or reduce oscillations in NGND. At time t₃,the transistor 421 may be turned on (e.g. the gate voltage shown in thetrance related to transistor 421 of FIG. 5 may transition high, turningthe transistor 421 on). When the transistor 421 is turned on, thevoltage NGND may be returned to ground as the capacitor 432 isdischarged through the transistor 421. Accordingly, the NGND signalfalls back to ground starting at time t₃.

At time t₄. the transistor 422 is turned off (e.g. the gate voltageshown in the trace related to transistor 422 of FIG. 5 may transitionlow, turning the transistor 422 off). The diode 443 may accordingly turnon, allowing current to flow, but since the transistor 421 remains on,the current may not charge the node NGND until the transistor 421 isagain turned off.

FIG. 6 is a schematic illustration of another fingerprint sensing systemin accordance with an embodiment. The fingerprint sensing system 600includes a fingerprint sensor chip 605, and a signal provider chip 610,which may be implemented using the fingerprint sensor chip 405 andsignal provider chip 410 of FIG. 4, respectively. Drive ring 612 andfingerprint receiving surface 614 are also shown. External componentsinductor 625 and diodes 642 and 643 are shown which may be used by thesignal provider chip 610 in providing a boosted voltage (e.g. NGND)during fingerprint scans. The inductor 625 and diodes 642 and 643 may beimplemented using the inductor 425 and diodes 442 and 443 of FIG. 4, andthe signal provider chip may include remaining components of theinductive boost circuit 500 of FIG. 5, for example. In FIG. 6, attentionis drawn toward the power domains that may be used for each of therespective chips and result from the operation of the booster circuit inthe signal provider chip 610.

For example, two domains are shown GND-referenced domain 650 andNGND-referenced domain 652. In the GND-referenced domain, operations mayoccur using a system ground (GND). In the NGND-referenced domain,operations may occur using a boosted ground (NGND). The boosted groundsignal (e.g. NGND) may only be boosted at particular times, such asduring a scan. Moreover, as described above, the boosted ground signal(e.g. NGND) may be modulated such that it has a frequency, whichfrequency may equal a frequency of a clock signal used by the system(e.g. NDRV). By raising the ground potential used by the fingerprintsensor chip 605, signal-to-noise ratio of fingerprint scans may beimproved without impact or with minimal impact to a user experience(e.g. tingling of the finger may not be induced since the boostedvoltage is not provided to the drive ring 612, and therefore a user'sfinger). Furthermore, the fingerprint receiving surface 614 may be madeof an insulating material preventing the finger from being in contactwith the sensor chip 605 and affected by the boosted voltage.Accordingly inputs to the fingerprint sensor chip 605 may be referencedagainst NGND, and not system ground.

Although embodiments have been described herein with respect toparticular configurations and sequences of operations, it should beunderstood that alternative embodiments may add, omit, or changeelements, operations and the like. Accordingly, the embodimentsdisclosed herein are meant to be examples and not limitations.

We claim:
 1. An electronic device, comprising: a fingerprint sensorcomprising a sensor pad and a drive ring adjacent to the sensor pad; anda modulating circuit operably connected to the sensor pad and the drivering, wherein the modulating circuit is adapted to control a modulatedsignal received by the sensor pad while maintaining a drive signalreceived by the drive ring substantially at a constant signal level. 2.The electronic device as in claim 1, wherein one signal level of themodulated signal comprises a low signal level.
 3. The electronic deviceas in claim 1, wherein one signal level of the modulated signalcomprises a high signal level.
 4. The electronic device as in claim 3,further comprising an inductive boost circuit operably connected to thesensor pad and the modulating circuit and adapted to boost the inputsignal to the high signal level.
 5. The electronic device as in claim 3,wherein the high signal level is greater than a power supply signallevel.
 6. The electronic device as in claim 1, further comprising ahousing surrounding the sensor pad and the drive ring, wherein thesensor pad is disposed below a button operably connected to the housing.7. The electronic device as in claim 1, further comprising a housingsurrounding a display surface, the sensor pad and the drive ring,wherein the sensor pad is disposed below the display surface.
 8. Theelectronic device as in claim 1, wherein the modulating circuit isincluded in the fingerprint sensor.
 9. The electronic device as in claim8, wherein the sensor pad and the drive ring are disposed below adielectric material.
 10. The electronic device as in claim 8, whereinthe fingerprint sensor comprises a capacitive fingerprint sensor.
 11. Afingerprint sensing system, comprising: a sensor pad; a drive ringadjacent to the sensor pad; and a modulating circuit operably connectedto the sensor pad and the drive ring, wherein the modulating circuit isadapted to control a modulated signal received by the sensor pad whilemaintaining a drive signal received by the drive ring substantially at aconstant signal level.
 12. The fingerprint sensing system as in claim11, wherein one signal level of the modulated signal comprises a lowsignal level.
 13. The fingerprint sensing system as in claim 11, whereinone signal level of the modulated signal comprises a high signal levelthat is greater than a power supply signal level.
 14. The fingerprintsensing system as in claim 13, further comprising an inductive boostcircuit electrically connected to the sensor pad and the modulatingcircuit and adapted to provide the high signal level.
 15. Thefingerprint sensing system as in claim 11, further comprising a housingsurrounding the fingerprint sensing system, wherein the sensor pad andthe drive ring are disposed below a button operably connected to thehousing.
 16. The fingerprint sensing system as in claim 11, furthercomprising a housing surrounding a display surface and the fingerprintsensing system, wherein the sensor pad and drive ring are disposed belowthe display surface.
 17. A method for operating a fingerprint sensorthat includes a sensor pad comprising a plurality of sensing elementsand a drive ring adjacent to the sensor pad, the method comprising:obtaining a first signal from at least one sensing element when an inputsignal to the sensor pad is in a first state and an input signal to thedrive ring is in a low state; obtaining a second signal from the atleast one sensing element when the input signal to the sensor pad is ina different second state and the input signal to the drive ring issubstantially maintained at the low state; and constructing an image ofat least a portion of a fingerprint based at least partially on thefirst and second signals.
 18. The method as in claim 17, whereinobtaining a first signal from at least one sensing element when an inputsignal to the sensor pad is in a first state and an input signal to thedrive ring is in a low state comprises obtaining a first signal from atleast one sensing element when an input signal to the sensor pad is at alow voltage level and an input signal to the drive ring is at a systemground voltage level.
 19. The method as in claim 18, wherein obtaining asecond signal from the at least one sensing element when the inputsignal to the sensor pad is in a different second state and the inputsignal to the drive ring is substantially maintained at the low statecomprises obtaining a second signal from the at least one sensingelement when the input signal to the sensor pad is at a high voltagelevel exceeding a power supply voltage level and the input signal to thedrive ring is substantially maintained at the system ground voltagelevel.
 20. The method as in claim 19, further comprising boosting theinput signal to the high voltage level using an inductive boost circuitprior to obtaining the second signal from the at least one sensingelement.